Nonagon (9 sides)

Outer radius r_o
Outer radius r_o
Number of sides 9
Side length a_9  = 2*r_o*sin(pi/9)
Area 9/4*a*a/tan(Pi()/9)
sum of sides 9*a_9
Outer angle 360°/9
Radius of inner circle r_i=a/(2*tan(36°))
Diagonal d=sqrt((a_9/2)^2+h^2)
Height h = r_o  + r_i
Sum of angles 1260°